Project Summary/Abstract The human population is increasingly at risk of developing chronic diseases, such as obesity, atopic dermatitis and inflammatory bowel disease, which together affects millions of people, thus representing a huge economic burden. One of the major contributing factors to these diseases is epithelial barrier dysregulation. The Ah receptor (AHR) is emerging as a major factor in the maintenance of immune surveillance and integrity of barrier tissues (e.g. skin, intestinal tract). However, our knowledge of AHR function is often confusing and at times appears contradictory. This proposal will pursue the innovative theory that the AHR is a central regulator of host barrier homeostasis, and the localized response to commensal and pathogenic microbes upon tissue damage leads to AHR activation and enhanced epithelial repair and barrier function. We have established an innovative multi-disciplinary team of collaborators with the intention of providing a comprehensive understanding of the physiological and toxicological routes of ligand stimulation, modes of activity and targets of AHR activation within epithelial barriers (e.g. intestinal mucosa and skin). I have been studying the AHR since 1984 and have made a number of unique and seminal contributions to our understanding of the physiologic and toxicologic functions of the AHR, thus I believe I am well qualified to lead this effort. Discoveries from the Perdew laboratory include complete characterization of the subunit composition of the AHR complex, and the determination that the liganded AHR works with inflammatory transcription factors to mediate cytokine/chemokine/growth factor signaling. We propose here to also examine the AHR as a potential therapeutic target for a number of chronic diseases. However, excessive or sustained activation of the AHR can lead to a variety of toxicities, so we plan to reconcile these opposing concepts. Humans are exposed to complex mixtures of AHR ligands from the diet, through microbiota metabolism, endogenous metabolism, and environmental contamination. Thus, there is a need to better understand the activities of this enigmatic receptor, the influence of different classes of ligands, and the appropriate level of AHR activation required to maintain health. Project 1 will identify and characterize proteins that interact with the AHR/selective AHR modulator (SAhRM) complexes. Project 2 proposes to identify and assess the significance of dietary/bacterial AHR ligands that are major sources of AHR activity. The identification of naturally occurring AHR ligands in the diet or synthesized by microbes in the gut will provide insights concerning the homeostatic role of the AHR and the risk of low-level exogenous ligand (e.g. TCDD) exposure. Project 3 will examine the ability of natural, exogenous, and synthetic AHR ligands to modulate barrier function and immune signaling in intestinal models. Project 4 will examine the ability of natural, exogenous, and synthetic AHR ligands to enhance barrier function in skin models. This highly innovative proposal will explore and validate a model for AHR function that unifies many diverse observations and will lead to important insights into the role of the AHR in disease processes.